Skin reflectance‐spectra and colour‐value dependency on measuring‐head aperture area in ordinary reflectance spectrophotometry and tristimulus colourimetry

Background/aims:  Measurement of skin colour has become increasingly popular in the study of dermatology with the increased availability of portable instruments. However, different instruments have been reported as giving different results from measurement of the same skin region. The aim of this study was to examine the effects of differences in measuring‐head aperture area on skin reflectance spectra and colour values. Methods:  We measured both reflectance spectra and CIE‐L*a*b* values of the skin in five different anatomical regions on 10 subjects using two MINOLTA reflectance spectrometers that were identical apart from the aperture area of the measuring heads (diameters: 5 and 11 mm). For comparison, data were also obtained from a skin‐coloured tile. Results:  Skin reflectance values measured with the wider‐aperture instrument were higher than those measured with the other, irrespective of anatomical location. The differences between the two were near zero at an incident light wavelength of 400 nm, but increased to around 10% of the reflectance value at 700 nm, increasing exponentially with incident light of increasing wavelength. Skin colour was observed to be brighter, redder and yellower, in CIE‐L*a*b* expression, when measured with the wider‐aperture instrument. The differences between measurements obtained from the skin‐coloured tile were much smaller. Conclusion:  Skin reflectance and colour values measured with reflectance instruments are not absolute data but depend on the aperture area of the measuring head. This is probably due to variations in the proportion of longer‐wavelength light reflected from the skin and collected by the instrument.